Re: [PATCH v4 07/10] x86: narrow out of bounds syscalls to sys_read under speculation

[Linus Torvalds]

On Tue, Feb 6, 2018 at 11:48 AM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>
> Just to clarify, when you say "this patch" you mean:
>
>      2fbd7af5af86 x86/syscall: Sanitize syscall table de-references
> under speculation
>
> ...not this early MASK_NOSPEC version of the patch, right?

I suspect not. If that patch is broken, the system wouldn't even boot.

That said, looking at 2fbd7af5af86, I do note that the code generation
is horribly stupid.

It's due to two different issues:

 (a) the x86 asm constraints for that inline asm is nasty, and
requires a register for 'size', even though an immediate works just
fine.

 (b) the "cmp" is inside the asm, so gcc can't combine it with the
*other* cmp in the C code.

Fixing (a) is easy:

  +++ b/arch/x86/include/asm/barrier.h
  @@ -43 +43 @@ static inline unsigned long
array_index_mask_nospec(unsigned long index,
  -                       :"r"(size),"r" (index)
  +                       :"ir"(size),"r" (index)

but fixing (b) looks fundamentally hard. Gcc generates (for do_syscall()):

        cmpq    $332, %rbp      #, nr
        ja      .L295   #,
        cmp $333,%rbp
        sbb %rax,%rax;   #, nr, mask

note how it completely pointlessly does the comparison twice, even
though it could have just done

        cmp $333,%rbp
        jae      .L295   #,
        sbb %rax,%rax;   #, nr, mask

Ho humm. Sad.

              Linus